Scannable mirror arrangement for an interferometer

ABSTRACT

A scannable mirror employs a mirror movable in an optical waveguide. The optical waveguide may be fluid filled and the mirror may be moved by an electromagnetic or electrostatic motor.

This Application claims the benefit of U.S. Provisional Application Ser.No. 60/443,294 filed Jan. 29, 2003.

The present invention relates to a scannable mirror for aninterferometer.

Optical spectroscopy is a sensitive and selective method of detectingand identifying molecules. It is rapid and requires little or no samplepreparation, and is amenable to continuous and real-time monitoring.Optical spectroscopy has many applications, including the detection ofchemical and biological materials, the determination of a person'smetabolic status, the monitoring of drinking water purity, and otheranalytical applications. Known optical coherence interferometry utilizesa mirror that scans back and forth rapidly (e.g., at 30-100 Hz) over asmall distance (e.g., about 1 mm or less), but is not susceptible tobeing used in a miniature instrument. In spite of its power, opticalspectroscopy is not widely used because spectrometers with usefulresolution are laboratory instruments that are too big, too fragile, andtoo sensitive to vibration, to be utilized in uncontrolled environments,such as might be encountered if the instrument were to be taken into thefield. Moreover, a cryogenically-cooled detector is often required,which presents a problem for use outside of the laboratory.

Interferometric Raman spectrometry has been proposed with single-modeoptical fiber elements, e.g., see H. I. Heaton, “Interferometric RamanSpectrometry with Fiber Waveguides,” Applied Optics, Vol. 36, No. 27, 20Sep. 1997, pages 6739-6750. Single-mode optical fibers, however, tend tobe more expensive and more sensitive to physical effects than aremulti-mode optical fibers, and the proposed instrument is reported asnot yet practical. Problems of scanning linearity and reproducibilityare reported for the as yet laboratory-grade system. Id. at page 6749.In addition, the use of single-mode optical fiber avoids the modaldispersion caused by multi-mode optical fiber and the effect of themodal dispersion that tends to degrade the resulting spectrum of thesample being measured.

Thus, there is a need and desire for a scannable mirror for an opticalinterferometer or spectrometer that is rugged and small, and that canprovide high spectral resolution. It would also be desirable thatscanning mirror arrangement be suitable for use in a hand-heldspectrometer.

To this end, the scannable mirror of the present invention comprises amirror disposed in an optical waveguide facing a first end thereof andmoveable therein toward and away from the first end thereof, and a motormoving the mirror in the optical waveguide toward and away from thefirst end of the optical waveguide,

BRIEF DESCRIPTION OF THE DRAWING

The detailed description of the preferred embodiments of the presentinvention will be more easily and better understood when read inconjunction with the FIGURES of the Drawing which include:

FIG. 1 is a schematic block diagram of an example embodiment of aspectrometer including an interferometer and scanning mirror inaccordance with the present invention;

FIGS. 2A and 2B are cross-sectional schematic diagrams of an exampleembodiment of a scanning mirror for the example embodiments of FIG. 1;

FIGS. 3A and 3B are cross-sectional schematic diagrams of twoalternative example embodiments of the scanning mirror for the exampleembodiment of FIG. 1;

FIG. 4 is a schematic diagram of an example embodiment of a scanningmirror employing differentially variable lengths of optical fiber;

FIG. 5 is a schematic flow diagram of a deconvolution process useful inaccordance with the invention;

FIG. 6A is a graphical representation of a typical intensity vs. wavenumber spectrum produced by the interferometer described herein; and

FIG. 6B is an expanded detail of the reference response of the referenceline of FIG. 6A.

In the Drawing, where an element or feature is shown in more than onedrawing figure, the same alphanumeric designation may be used todesignate such element or feature in each figure, and where a closelyrelated or modified element is shown in a figure, the samealphanumerical designation primed or designated “a” or “b” or the likemay be used to designate the modified element or feature. It is notedthat, according to common practice, the various features of the drawingare not to scale, and the dimensions of the various features arearbitrarily expanded or reduced for clarity.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Fourier transform spectroscopy is a powerful method for obtaining theabsorption and Raman spectra of chemical compounds, and has advantagesover dispersive methods of spectroscopy including: Use of a singledetector rather than a detector array, simultaneous detection of allincident light rather than a small spectral band, high spectralresolution without spatial filtering, and readily adjustable spectralresolution. The second and third of the foregoing advantages tend togreatly enhance the signal-to-noise ratio of the spectrometer due to themore efficient use of the incident light.

Infrared absorption spectroscopy and Raman spectroscopy are two relatedspectroscopic techniques that give very similar information aboutmolecular structure. Both determine vibrational frequencies of themolecule from about 500 to 5000 cm⁻¹ (i.e. from about 2 to 20 μmwavelength). While the present invention may be employed in bothinfrared absorption spectroscopy and Raman spectroscopy, as well asother methods of spectroscopy, the inventor has recognized that Ramanspectroscopy has several features that make a small or even miniatureRaman spectrometer practical:

(1) The light source for Raman spectroscopy is a single collimated pumplaser 20 rather than a broadband infrared source, as in infraredspectroscopy;

(2) The Raman signal is in a relatively narrow optical band surroundingthe pump laser wavelength;

(3) Raman spectroscopy can be done with a room temperature silicondetector 50 rather than requiring a mid- to far-infrared detector thatrequires cooling (e.g., cryogenic cooling) or has low detectionsensitivity; and

(4) Raman spectroscopy signal increases with increasing concentrationfrom a zero intensity background, whereas infrared absorptionspectroscopy signal decreases with increasing concentration from a largebackground that is related to the illumination intensity.

(5) The detection of low concentrations by Raman spectroscopy is easier,e.g., because of lower shot noise from the background, and it is lesssusceptible to interference from water in the sample, particularly ofbiological samples which have high water content.

These attributes may be utilized to enable the reduction of the sizeand/or the power consumption of the spectrometer, as is desirable for aportable instrument.

To reduce the size further, the light dispersion element, which iscommonly a grating in a laboratory spectrometer, is replaced by ascanning mirror 100 in a Michelson interferometer to do Fouriertransform Raman spectroscopy. In a Fourier transform spectrometer 10,the light intensity is measured as a function of time as the mirror 100in one arm of the interferometer is scanned back and forth. This signalis numerically Fourier transformed to give the spectrum. Transformspectroscopy has inherent signal-to-noise advantages that are useful fora small and/or portable instrument. A multiplex advantage obtains inthat all wavelengths are measured simultaneously with a single detector,and a throughput advantage obtains in that the spectral resolution canbe increased without using a slit to block a portion of the light.

FIG. 1 is a schematic block diagram of an example embodiment of aspectrometer 10 including an interferometer and scanning mirror inaccordance with the present invention. The spectrometer 10 is a Ramantype interferometer in which the light produced by laser 20 impingesupon sample S and the Raman scattered light spectrum therefrom isdetected 50 and analyzed. Spectrometer 10 includes a scanning Michelsoninterferometer. Optical fiber coupler 40 couples part of the light beamfrom sample S to a fixed mirror 44 and part to a scanning mirror 100,110. Spectrometer 10 employs a scanning mirror 100, 110 according to theinvention and performs analysis of the light spectrum from sample Sincluding Fourier transformation.

The light source for Raman pumping is a diode laser 20. The wavelengthfor Raman spectroscopy is not critical and is determined factors such assample absorption and available power. Raman pump laser diodes are oftenin the near infrared part of the spectrum because of theseconsiderations. For high resolution, the laser 20 should be adistributed feedback (DFB) laser that operates at a single wavelength,e.g., 800-900 nm. Output signal from laser back facet monitor 66 can beused by control and data analysis electronics 60 for controlling thelaser 20. The light output 26 of the laser 20 is coupled via an opticalisolator 22 into an optical fiber 24 wherein it is guided into anoptical circulator 30 or a wavelength division multiplexer 30 and to theend of sensing optical fiber 32.

Lens 34 images the pump light 26 onto the sample S as it leaves theoptical fiber 32, with sample S preferably being at the focus of lens34. Because of the narrow optical bandwidth of the Raman light,chromatic aberration of lens 34 can be neglected and the same lenscollects the Raman scattered light 26′ and couples it back into opticalfiber 32. The collection efficiency is determined by the solid anglesubtended by lens 34 in the image (sample S) side. Lens 34 may be anoptical fiber graded index (GRIN) lens.

Raman scattered light 26′ will always be collected because the pumplight 26 and the Raman scattered light 26′ follow the same path.Misalignment of lens 34 will decrease the intensity of the pump light 26in the sample S, but Raman scattered light 26′ will still be collected.The collected Raman light 26′ as well as some of pump light 26 that isRayleigh scattered or reflected by the sample S is guided via opticalfiber 36 to the scanning mirror 100 interferometer by the circulator orwavelength division multiplexer 30. The pump light 26 directed back intothe interferometer 10 is utilized to determine the fiber-waveguideresponse function, while the Raman scattered light 26′ contains chemicalinformation about the sample S.

A portion of the reflected light 26′ is coupled by coupler 40 to andfrom fixed mirror 44 via optical fiber 42 and another portion ofreflected light 26′ is coupled by coupler 40 to and from a scannable orscanning mirror 100 via optical fiber 46. For maximum signal, coupler 40should split the light 26′ equally between optical fibers 42 and 46 tomirrors 44 and 100, respectively. Light reflected from fixed mirror 44interferes with light reflected from scanning mirror 100 and is coupledby coupler 40 and optical fiber 52 to detector 50. Interferences areproduced in coupler 40 and are coupled to detector 50 via optical fiber52 for various relative differences in the distance reflected light 26′travels between coupler 40 and fixed mirror 44 on the one hand andbetween coupler 40 and scanning mirror 100 on the other hand. Detector50, e.g., a silicon detector, converts the light impinging thereon to anelectrical signal which is amplified by preamplifier 54 and applied tocontrol and data analysis electronics 60 wherein it is analyzed asdescribed below to determine the characteristics of sample S. Both fixedmirror 44 and mirror end surface 110 of movable rod 112 are polished andcoated with a high reflectance optical coating, e.g., a metallic coatingsuch as aluminum, silver, gold.

The Michelson interferometer of FIG. 1 is preferably employs multimodeoptical fibers and/or waveguides 24, 32, 36, 42, 46, 52, and a 3-dBfiber coupler 40. The scan range of scannable mirror 110 determines theresolution of the resulting spectrum, with a longer scan range providinga higher spectral resolution. The spectral range is ultimatelytheoretically limited by the smallness of the size of the steps of thescan, but in practice is typically limited by the capability of othercomponents, typically the spectral response of detector 50.

While the light paths of laboratory spectrometers are through free-space(free-space optics), the preferred embodiments of spectrometer 10,employs light paths that are within optical fibers and/or opticalwaveguides and/or other optical components (guided-wave optics), therebyto enable reduction of the size of the spectrometer and to make it morephysically robust, where such is desired. Fiber 3-dB coupler 40 has oneleg as an input port receiving fiber 36, and another as an output portto fiber 52 coupled to detector 50. A third leg of coupler 40 is fiber42 which is cut, polished and high-reflectance coated to serve as afixed mirror 44, and a fourth is a port to fiber 46 to scanning mirror100, 110.

With regard to the spectrometer 10, because the Raman signal 26′ has arelatively narrow optical bandwidth, the entire Raman spectrum will beguided and properly split by the 3-dB coupler 40. The 3-dB coupler 40,which may be a fiber circulator or a wavelength division multiplexer,separates the counter propagating pump and Raman beams. An ordinarypower splitter could be employed if the additional coupling loss thereofis acceptable. Desirably, coupler 40 has sufficient crosstalk so thatsome of the Raman excitation from fiber 36 is coupled to detector 50 foruse in determining the system spectral response, as is described below.Besides reducing the size of the spectrometer 10, this arrangementeliminates the need to make and to maintain critical alignments andhelps to provide a very robust instrument.

Operation of interferometer 10 is controlled by control and analysiselectronics 60. Specifically, electronics 60 includes an electronicprocessor and produces control signals that are coupled to laser 20 bylaser driver 64 for causing laser 20 to produce pumped light 26, andreceives feedback signals monitoring the laser 20 back facet via monitor66. The signal produced by detector 50 is the Fourier transform of thespectrum and electronics 60 generates the inverse Fourier transformthereof to produce the actual spectrum. In addition, electronics 60produces control signals for scanning scannable mirror 100 that arecoupled to scanning mirror 100 via mirror driver 62 to cause thescanning thereof in a desired manner. Example scanning mirror 100includes a mirror face 110 that is moved by forces from a movable magnet122 which is mechanically moved by a motor 120, as is described indetail below, as are alternative embodiments thereof.

Control and data analysis electronics 60 receives as an input controlsignals for controlling operation of interferometer 10 and provides dataout signals representative of analyzed measured data for sample S. Apower conditioning and distribution device 70 receives electric inputpower, conditions the input power to the voltages and currents requiredby the various elements of interferometer 10, and distributesconditioned power thereto.

FIGS. 2A and 2B are plan view and end view cross-sectional schematicdiagrams of an example embodiment of a scannable or scanning mirror 100useful in the example embodiments of FIG. 1. Scanning mirror 100 is animportant component of Fourier transform spectrometer 10 because thescan range of scanning mirror 100 determines the spectral resolution ofspectrometer 100. A scan range (stroke) of about 1 cm providesreasonable spectral resolution. While a longer stroke could easily beprovided and would provide greater spectral resolution, such resolutionis typically not necessary for typical liquid or solid samples.

Scannable scanning mirror 100 is the most difficult component ofinstrument 10 to miniaturize, and is preferably implemented by amicro-electro-mechanical structure (MEMS) waveguide of which variousembodiments are described herein in detail, e.g., in relation to FIGS.2A, 2B, 3A, 3B, and 4. Besides reducing the size of the spectrometer,this arrangement eliminates the need to make and to maintain criticalalignments and makes for a very robust instrument.

Details of a generic MEMS-waveguide scanning mirror 100 are shown inFIGS. 2A and 2B. Scanning mirror 110 is provided by a polished andmetalized end 110 of a rod 112 that is movably disposed in aliquid-filled channel 104 providing an optical waveguide in a glasssubstrate base 102. The width of channel 104 is preferably comparable tothe diameter and/or cross-sectional dimension of the core 46 a ofoptical fiber 46. A glass cover 108 is attached, e.g., anodic bonded, tothe glass substrate base 102 to provide an upper waveguide clad and tocontain the liquid in channel 104 therein. The index of refraction ofthe liquid is preferably slightly higher than that of the glass 102, 108so that it forms an optical waveguide in channel 104.

Optical fiber 46 is, e.g., butt-coupled to the liquid waveguide 104 andsealed by sealant 114 to encapsulate the liquid therein. Typically,channel 104 is counterbored to receive the cladded fiber core 46 a, 46b. Proper selection and control of the liquid waveguide 104 dimensionsand of the indices of refraction of the materials, permits reflection atthe fiber-liquid interface 46 a, 104 to be made acceptably small.Substrate 102 and cover 108 may be of any suitable optically uniformglass, preferably of the same index of refraction, and a siliconeoptical fluid is preferred to fill channels 104, 106. Preferably, theindex of refraction of the liquid filling channel 104 is about the sameas that of fiber core 46 a, and the index of refraction of the glass ofsubstrate 102 and cover 108 is about the same as that of fiber cladding46 b, so as to minimize reflection at the interface of core 46 a and theliquid in channel 104.

Channel 104 waveguide and bypass channel 106 may be chemically orotherwise etched, laser ablated, sawn, cut, diamond sawn, hot pressed,or otherwise formed, in glass substrate 102. Presently, diamond sawingfor channel 104 and chemical etching for channel 106 are thought to bepreferred. While a channel having a circular cross-section matching thatof the core 46 a of fiber 46 would be preferred for optical waveguideperformance, an exact match is not necessary, and other considerationsmay make another shape and/or size desirable. Another suitablecombination is for substrate 102 and cover 108 to be of PYREX® glass(which has an index of refraction of about 1.47) with benzene (index ofrefraction of about 1.50) as the fluid filling channel 104.

The length of the path over which light 26′ travels is changed by movingrod 112 (and thus the polished mirrored face 110 thereof) longitudinallyin liquid-filed channel 104. Rod 112 and channel 104 preferably have adiameter/dimension that is similar to the diameter of core 46 a ofoptical fiber 46, e.g. about 50-100 μm. Core 46 a is surrounded byreflective cladding 46 b which is in turn surrounded by a jacket 46 cfor physical protection. Typically, rod 112 is about 50 μm indiameter/dimension, and channel 104 is slightly larger, e.g., 55-60 μm.Typically, core 46 a and rod 112 have a circular cross-section, whilechannel 104 has a rectangular, square or trapezoidal cross-section.Optical losses due to mismatch of the foregoing dimensions can becompensated, if necessary, by introducing optical losses into the fixedmirror 44 leg or into coupler 40.

Neither rod 112 nor channel 104 need be of circular cross-section, andit is advantageous if they are of different cross-sectional shapes sothat rod 112 does not fill channel 104, thereby to provide passagesalong rod 112 (i.e. between rod 112 and the walls of channel 104)through which the fluid in channel 104 may flow parallel to rod 112 asrod 112 moves longitudinally within channel 104. The viscosity of thefluid filling channel 104 directly affects the ease with which rod 112moves therein. It is noted, however, that while a rod 112 that moveseasily back and forth can be scanned with little power, a rod 112 thatis more difficult to move will tend to maintain alignment and will beless susceptible to movement caused by external vibration and otherforces. It is also desirable that the rod 112 and channel 104 have wellmatched cross-sections for keeping rod 112 properly oriented/alignedwithin channel 104 (i.e. close to coaxial therewith) and for obtaininghigh reflectivity from mirrored end 110.

In addition, to ease the movement of rod 112 in liquid-filled channel104, e.g., to avoid the resistance that could be caused by the liquidhaving to pass between the rod 112 and the walls of channel 104 as rod112 moves longitudinally therein, a bypass channel 106 may be providedto provide an alternative path for liquid to flow between one end ofchannel 104 and the other end thereof. Rod 112 may have a circularcross-section, or may be rectangular or octagonal or any other shapecompatible with the cross-sectional shape of channel 104. Typically,bypass channel 106 is of smaller cross-sectional dimension than ischannel 104.

The range of movement of mirror 110 relates to the resolution attainableand the step size within that movement range determines the spectralrange. Rod 112 can be moved back and forth in channel 104 with a travelof about 1 cm by the example drive methods described below. A longerstroke, e.g., greater than about 1 cm, which can easily be implemented,will yield higher spectral resolution, but may not be needed for liquidor solid samples S where a 1 cm stroke already provides a spectralresolution comparable to the line width. To obtain a spectral range of5000 cm⁻¹, which would include all Raman lines of interest for chemicalanalysis, the step size of the travel of rod 112 must be about 2 μm.During movement of rod 112, liquid circulates through the liquid bypasschannel 106. As a result, the frictional forces resisting the movementof rod 112 are made acceptably small.

FIGS. 3A and 3B are cross-sectional schematic diagrams of twoalternative example embodiments of a motor-driven scanning mirror 100for the example embodiment of FIG. 1.

In the motor driven scanning mirror 100 of FIG. 3A the scanning mirrorfacet 110 is a polished end of a ferromagnetic rod 112 that ismagnetically translated longitudinally in the liquid-filled waveguidechannel 104. An external, linear translation motor 120 is coupled totranslate a magnet 122 that magnetically couples to the ferromagneticrod 112. As the magnet 122 moves or oscillates back and forth adjacentglass cover 108 and parallel to liquid filled waveguide channel 104, therod 112 will follow magnet 122 if it is made of a magnetic or ferrousmaterial or other ferromagnetic material. Typically, cover 108 is about0.5 mm or less in thickness.

Rod 112 is typically of a nickel or nickel steel material, but may bemade of, coated with or have embedded therein, any magnetic orferromagnetic material. Movement of magnet 122 may be any suitablemotive means, e.g., such as a solenoid, a motor, a lead screw or astepping motor. Small, low cost, linear translation motors 120 with theneeded step size and range of travel are readily available, and arecommonly utilized as head drive motors for computer hard drives and CDplayers.

In the motor driven scanning mirror 100 of FIG. 3B, the scanning mirrorfacet 110 is a polished end of a dielectric rod 112′ that iselectrostatically translated longitudinally in the liquid-filledwaveguide channel 104. Dielectric rod 112′ is driven with a MEMS motormicro-fabricated linear stepping motor 120′ having a stator comprisingelectrodes 116 on glass cover 108 and having a rotor comprisingcircumferential conductive ring electrodes 114 on dielectric rod 112′.Rod 112 is preferably a glass or ceramic rod, and electrodes 114,116 arepreferably gold over a titanium base.

The combination of rectangular stator electrodes 116 and circumferentialrotor electrodes 114 on movable dielectric rod 112 provide a linearstepping motor formed of micro-fabricated elements. Electrodes 116 aretypically energized individually to electrostatically pull the neareststripe electrode 114 into alignment therewith, to provide a vernierpositioning control. Electrodes 116 are typically disposed at adifferent pitch than are electrodes 114. The pitch of electrodes 116 istypically greater than is the pitch of electrodes 114, and the axialwidth or dimension of electrodes 116 is typically greater than is theaxial width of stripe electrodes 114.

Regardless of the drive arrangement utilized to move rod 112, 112′ inchannel 104, the longitudinal motion of rod 112, 112′ can be monitoredinterferometrically utilizing either the Raman pump laser 20 or aseparate laser so that viscous drag on the rod 112, 112′ motion can betaken into account. Thus, the linearity and/or accuracy of the movementof mirror 110 need not be highly controlled.

FIG. 4 is a schematic diagram of another example embodiment of ascanning mirror 100 employing differentially variable lengths of opticalfiber 42, 46. Each of optical fibers 42, 46 has about the same longlength and is wound around a spool, reel or other form 124 many times,and each has a polished mirror end or face 44, 144, respectively. Form124 comprises two separate forms 124 a, 124 b on which the respectivelengths of fibers 42, 46 are wound. Forms 124 a, 124 b have a physicalcharacteristic that changes responsive to an electrical signal appliedthereto by driver 62′. The electrical signal is applied so as to causeone of forms 124 a, 124 b to increase in physical size, therebystretching the length of the one of fibers 42, 46 wound thereon, and theother of forms 124 a, 124 b to decrease in physical size, therebyallowing the length of the one of fibers 42, 46 wound thereon to shrink.

Suitable materials for forms 124 a, 124 b include materials with arelatively high coefficient of thermal expansion so that driver 62′applying an electrical signal thereto produces heat (e.g., as in-aresistance heater) which raises the temperature of form 124 a, 124 bcausing it to expand. Other suitable materials include piezoelectric andelectrostrictive materials that similarly change in physical dimensionresponsive to the electrical signal applied thereto. In practice,opposing electrical signals are applied simultaneously to both of forms124 a, 124 b so that a differential change in lengths of fibers 42, 46obtains.

To scan the mirrors 44, 144, the electrical signal to one spool 124 a isinitially at a relatively low value so as to decrease the length offiber 42 thereon and the signal to the other spool 124 b is initially ata relatively high value so as to increase the length of fiber 46thereon, thereby to differentially change the difference between thelengths of the respective light paths in fiber 42 and fiber 46 to a highvalue in a first sense (e.g., fiber 42 is shorter than fiber 46).Scanning is provided as the electrical signal applied to form 124 a isincreased and the electrical signal applied to form 124 b is decreased.At the end of the scan, the electrical signal to spool 124 a is at arelatively high value so as to increase the length of fiber 42 thereonand the signal to the other spool 124 b is at a relatively low value soas to decrease the length of fiber 46 thereon, thereby to differentiallychange the difference between the lengths of the respective light pathsin fiber 42 and fiber 46 to a high value in a second sense opposite tothe first sense (e.g., fiber 42 is longer than fiber 46).

Regardless of the drive arrangement utilized for changing the relativesizes of forms 124 a, 124 b, and therefore the lengths of fibers 42, 46,the difference in lengths thereof can be monitored interferometricallyutilizing either the Raman pump laser 20 or a separate laser so thatstatic differences in the lengths of fibers 42, 46, e.g., as may beproduced by cutting and manufacturing tolerances, can be taken intoaccount.

As thus far described, spectrometer 10 may employ either single-modeoptical fibers or multimode optical fibers, as may be desirable in agiven application. Preferably, however, spectrometer 10 employsmultimode optical fibers 24, 32, 36, 42, 46, 52 and waveguides 30, 40,104, because the use of multimode fibers and waveguides tends toincrease the efficiency of light coupling into the fiber and to simplifythe construction of the scanning mirror 110. A penalty for usingmultimode fibers 24, 32, 36, 42, 46, 52 is modal dispersion, which willlimit the spectral resolution to about 10-100 cm⁻¹ absent correction.Modal dispersion arises because the photons travel at differentvelocities in multimode fibers, which tends to broaden out, and possiblyobscure, the peaks in the spectrum.

Spectrometer 10 cannot differentiate or distinguish between a change inphase velocity and a change in wavelength in a multimode system, and sothe observed spectrum with monochromatic light (laser 20) in a multimodesystem will be the same as some polychromatic spectrum in a single modesystem. Thus, the effect of modal dispersion is to convolute the Ramanspectrum with this polychromatic spectrum (which is called thefiber-waveguide response function herein). The result of thisconvolution with a typical fiber-waveguide response function is shown inFIG. 6A. The convoluted spectrum appears to be slightly degraded, but itstill retains major spectral features. While the spectrometer may beuseful as is, the inventor has discovered how to correct for the modaldispersion in a multimode fiber and/or multimode waveguide system.

To this end, the inventor has recognized that the fiber-waveguideresponse function can be extracted from the spectrum, e.g., from theRaman spectrum near the origin, and can be utilized to deconvolute thespectrum, thereby to obtain the advantage of multi-mode fibers whileavoiding their principal disadvantage. The fiber-waveguide responsefunction typically has a spectral width of about 20 cm⁻¹, which is muchless than the Stokes shift in the Raman spectrum. The spectrum in thevicinity of the single wavelength Raman pump 20 line is just thefiber-waveguide response function and can be numerically extracted,e.g., by electronics 60.

Among the benefits of obtaining the fiber-waveguide response function insitu is that system drifts due to temperature change, and/or componentaging, and/or bending or repositioning of an optical fiber, and/orchanges in coupling of light into the optical fiber, are automaticallycompensated. Moreover, changes resulting from handling (and even somemis-handling) the spectrometer during measurements, such as bending of afiber or altering of the light coupling into the fiber, may also beautomatically compensated. This method, which is described in detailbelow, not only improves the spectral resolution by removing the effectsof modal dispersion, but also greatly enhances the spectrometerperformance in the uncontrolled environments that a non-laboratoryinstrument, such as a hand-held instrument, will see service.

FIG. 5 is a schematic flow diagram of a deconvolution process, ormethod, 200 useful in accordance with the invention, which is describedin relation to FIGS. 6A and 6B. FIG. 6A is a graphical representation ofan example Fourier transform Raman spectrum 300, 310 intensity vs. wavenumber produced by the interferometer 10 described herein, and FIG. 6Bis an expanded detail of the response function of the reference line 305at or near the origin of spectrum 300 of FIG. 6A. Because the modaldispersion introduced by the multimode optical fiber(s) and/orwaveguide(s) convolutes the response spectrum of the interferometer, theresulting convoluted spectrum 300 is somewhat degraded from what a fullresolution spectrum 310 would be. The degradation is manifest as aspreading of the spectrum peaks, including that of the laser referenceline 305 produced by laser pump source 20 which intrinsically has a verysharp, narrow peak.

In the expanded depiction of reference line 305 it is seen that thebroadened laser reference line comprises a response function 305 havinga relatively narrow spectrum of plural peaks that arise in situ due tothe modal dispersion. Even with the spreading of the spectrum 300 peaks,the first peak of each of spectrum 300 and spectrum 310 is only about 20cm⁻¹ wide and is still far removed from (to the right of) the referenceline 305, typically spanning about 200-5000 cm⁻¹. In other words, thereis a significant region, e.g., wave numbers of about 20-200 cm⁻¹,between the origin and the first peak of data spectra 300, 310, in whichthere is no data, and so reference line 305 may be utilized forconvoluting the data spectrum 300 to obtain the deconvoluted spectrum310 which is substantially free of the effects of the modal dispersionintroduced by the multimode optical fibers.

Process 200 comprises acquiring 205 a Fourier transform Raman spectrumi(t), which is also known as an interferogram i(t), in a scanninginstrument 10 as described. Interferogram i(t) is a function of time tas measured. Fourier transformation 210 is applied to the interferogramto obtain a convoluted 215 Raman spectrum I(ν) 300 which is a functionof wave number ν. Low pass filtering 220, e.g., numerical filtering withabout a 10-100 cm⁻¹ bandwidth, separates the fiber/waveguide responsefunction H(ν) 305 from the spectrum 300 and the response function H(ν)is inverse Fourier transformed 230 to obtain the fiber/waveguideresponse interferogram h(t) 235.

Then, interferograms i(t) and h(t) are deconvoluted 240, e.g., bydividing i(t) by h(t) and Fourier transforming the ratio functioni(t)/h(t), to obtain a deconvoluted 245 Raman interferogram r(t) whichis in turn Fourier transformed 250 to obtain a deconvoluted 255 Ramanspectrum R(ν) 310 Because the same multimode distortions affect theinterferogram and the response function, the foregoingconvolution-deconvolution process removes the effects thereof as theyare at the time each measurement is made. As a result, the instrumentaccuracy is relatively unaffected by changes in and to the componentscomprising the instrument.

Fourier transformations and inverse Fourier transformations aretypically provided by numerical operations, as is numerical filtering,e.g., low-pass filtering, such as in an electronic processor 60 thatperforms control and data analysis functions.

Where a separate, e.g., auxiliary source is utilized to provide areference line for processing the spectrum, the inverse Fouriertransform of the system response function is obtained directly (i.e. asat the conclusion of step 235 of FIG. 5. The directly obtained responsefunction is then deconvoluted 240 and Fourier transformed 250 asdescribed above.

It is noted that if any of the optical characteristics of the opticalfibers and/or waveguides change, e.g., due to temperature, aging,bending and/or stretching of the fiber or other physical and/ormechanical change, changes where light is coupled, changes at opticalinterfaces, and the like, the in situ laser reference response function305 distorts in response to the effects of such change, and so theconvolution process utilizing that response function automaticallyincludes such effects and compensates therefor. In addition, while thepump laser 20 is described herein as providing measurement also providesthe response function 305 utilized in the convolution processing, as istypical in Fourier transform Raman spectroscopy, another laser may beprovided to provide a reference line that is distorted by thecharacteristics of the multimode optical fiber(s) and/or waveguide(s)and is then utilized as the response function in the convolutionprocessing.

A laser or other light source is referred to herein as substantiallymonochromatic if its light output is at a single frequency or wavelengthor is over a bandwidth or range thereof that is sufficiently narrow asto produce a reference line in an interferogram or other measurementspectrum that is sufficiently separated from spectral data produced by asample being measured that the reference line can be utilized as aresponse function.

In summary, the Fourier transform Raman spectrometer described herein issuitable for miniaturization, and can be made small enough and robustenough to be embodied in a hand-held instrument that includes the laserlight source, all of the optics, and the control and analysiselectronics. The spectral resolution of such instrument is expected tobe 1 cm⁻¹ or better and the spectral range to be 0-5000 cm⁻¹. In otherwords, its performance is expected to be much better than anyconventional small spectrometer and to be comparable to a laboratoryspectrometer in terms of spectral resolution, spectral range andthroughput. Such instrument has utility for chemical and/or biologicalanalysis, chemical and other material identification, and metabolicmonitoring. Samples may be, e.g., in gaseous or liquid or solid form.

A typical hand-held spectrometer 10 can be provided in a package aboutthe size of a typical personal digital assistant (PDA). Typicalcomponents therefor include a laser source 20 and a detector 50 eachpig-tailed to a fiber 24, 52 and about 0.6 cm in diameter and less thanabout 5 cm long, a 3-dB coupler 40 and a circulator or multiplexer 30each about 0.5 cm in diameter and less than about 5 cm long, and anelectronics board for processor electronics 60 that is about 3 cm by 1cm. A scanning mirror assembly 100 providing a 1 cm stroke is about 5 cmby 1.5 cm by 1 cm if a MEMS motor 120′ is employed and about twice thatsize if a small linear translation motor 120 (as in a portable CDplayer) is employed. A laser diode 20 with a light output in the tens ofmilliwatts and operating at a wavelength of about 700-800 nm ispreferred, e.g., for sampling on tissue, however, it may be desirable tooperate such laser at a low duty cycle so as to keep the average poweroutput at a level that is safe for the human eye. While such low dutycycle operation would increase the time required to measure a sample, itwould result in a reduction of the power necessary to scan mirror(s)112, 122, 124. For high spectral resolution, laser 20 is preferably adistributed feedback (DFB) laser diode that operates at a singlewavelength.

As used herein, the term “about” means that dimensions, sizes,formulations, parameters, shapes and other quantities andcharacteristics are not and need not be exact, but may be approximateand/or larger or smaller, as desired, reflecting tolerances, conversionfactors, rounding off, measurement error and the like, and other factorsknown to those of skill in the art. In general, a dimension, size,formulation, parameter, shape or other quantity or characteristic is“about” or “approximate” whether or not expressly stated to be such.

While the present invention has been described in terms of the foregoingexemplary embodiments, variations within the scope and spirit of thepresent invention as defined by the claims following will be apparent tothose skilled in the art. For example, while a reflective illuminationof a sample S to produce a Raman spectrum is illustrated, the instrumentand/or scanning mirror described herein may be utilized where a sample Sis transmissively illuminated to produce an absorption spectrum.

Further, while magnetic and electrostatic means are described forimparting motion to rod 112 in channel 104, other means may also beemployed. Such other means might include thermal means, such as abimetallic or other thermally expansive element, and/or piezoelectricmeans, such as a piezoelectric element. Mirror 110 on rod 112 may be apolished end thereof or a mirror attached thereto.

Apparent movement of the mirror, e.g., by optically lengthening and/orshortening of the light path, may be effected by altering, e.g., eitherthermally or electrically, the index of refraction of the liquid thatfills channel 104 without moving the rod 112, so long as the index ofrefraction of the liquid remains higher than that of the glass 102, 108so as to maintain waveguide action of channel 104. While the extent ofthe apparent movement is quite limited, it could be utilized to providea position vernier for rod 112 in combination with the other means formoving rod 112 described herein.

Scannable (i.e. able to scan) or scanning are used interchangeablyherein to denote the feature of a mirror that may be moved to provide anoptical scanning function and the optical scanning function itself, asshould be clear from the context. While one mirror of the Michelsoninterferometer is described as scanning and the other as being fixed incertain embodiments herein, either one or both mirrors may be scanned inany embodiment. Where both mirrors are scanned, they are preferablyscanned in opposite directions, i.e. 180° out of phase with each other,so as to double the scan range and the resulting spectral resolution.Thus, respective scanning mirror assemblies 100 could be placed at therespective ends of both of fibers 42 and 46.

Lens 34 may be at the surface of a container for spectrometer 10, i.e.may be flush with or slightly raised or recessed, or may be at the endof an optical fiber cable extending therefrom. In addition, an opticalfiber cable could be placed with one end against lens 34 and the otherend against sample S, so as to make measurement for convenience or tomeasure a sample S in a confined or awkward location.

While certain electrical signals are described as being “high” or “low”values, it is understood that this may refer to magnitude of voltage orcurrent or power, or may refer to being of negative and positivepolarities, or may refer to being more negative or more positive.

What is claimed is:
 1. A scannable mirror arrangement comprising: asubstrate having an elongate channel therein, the channel having firstand second ends; an optical fiber having a first end abutting the firstend of the channel in said substrate; a rod disposed in the channel ofsaid substrate and moveable longitudinally therein, said rod having amirror surface on one end thereof facing the first end of said opticalfiber; an optical fluid filling said channel, whereby the fluid-filledchannel provides an optical waveguide; and a motor for moving said rodlongitudinally in the channel of said substrate, whereby the distancelight travels in the optical waveguide between the first end of saidoptical fiber and the mirror end of said rod changes in response to saidmotor moving said rod.
 2. The scannable mirror of claim 1 wherein saidsubstrate includes a base having the channel in a surface thereof and acover attached to the surface of said base having the channel therein.3. The scannable mirror of claim 1 wherein the channel in said substratecomprises a main channel in which said rod is disposed, and a secondchannel providing a passage between first and second ends of the mainchannel, whereby said optical fluid may flow in the second channelbetween the first and second ends of the main channel as said rod movestherein.
 4. The scannable mirror of claim 1 wherein said substrateincludes an optical glass having an index of refraction, and whereinsaid optical fluid has an index of refraction greater than the index ofrefraction of the optical glass.
 5. The scannable mirror of claim 1wherein said substrate includes an optical glass, and wherein saidoptical fluid includes a silicone fluid and/or benzene.
 6. The scannablemirror of claim 1 wherein the channel has a cross-sectional shape andsize and wherein said rod has a different cross-sectional shape and/or adifferent cross-sectional size.
 7. The scannable mirror of claim 6wherein the cross-sectional shape of said channel is one of rectangular,trapezoidal and circular, and wherein the cross-sectional shape of saidrod is circular.
 8. The scannable mirror of claim 1 wherein said opticalfiber has a cross-sectional shape and size and wherein the channel hasabout the same cross-sectional shape and size.
 9. The scannable mirrorof claim 1 wherein the channel of said substrate has a counterbore atthe first end thereon and wherein said optical fiber is disposed in thecounterbore of the channel.
 10. The scannable mirror of claim 1 whereinsaid motor further includes a stepping motor, a linear motor, atranslating motor, an electromagnetic motor and/or an electrostaticmotor.
 11. The scannable mirror of claim 10 wherein said motor includesa magnet moveable longitudinally adjacent the channel of said substrate,and wherein said rod is magnetic and/or ferromagnetic.
 12. The scannablemirror of claim 10 wherein said motor includes a plurality of electrodesspaced apart on said substrate along the channel therein, and whereinsaid rod is dielectric and includes a plurality of spaced apartelectrodes thereon.
 13. The scannable mirror of claim 12 wherein theplurality of electrodes on said substrate are spaced apart at a pitchgreater than that of the plurality of electrodes on said rod.
 14. Thescannable mirror of claim 1 wherein said optical fiber is a multimodeoptical fiber and wherein the fluid-filled channel of said substrateprovides a multimode optical waveguide.
 15. A scannable mirrorarrangement comprising: an optical fiber having an end abutting anoptical waveguide; a mirror disposed in the optical waveguide andmoveable therein toward and away from said optical fiber, wherein saidmirror faces the abutting end of said optical fiber; and motor means formoving said mirror in the optical waveguide toward and away from saidoptical fiber, whereby the distance light travels in the opticalwaveguide between the end of said optical fiber and said mirror changesin response to moving said mirror.
 16. The scannable mirror of claim 15wherein the optical waveguide comprises a substrate having a channel ina surface thereof and a cover attached to the surface of said substratehaving the channel therein, wherein the channel is filled with opticalfluid.
 17. The scannable mirror of claim 16 wherein the channel in saidsubstrate comprises a main channel in which said mirror is disposed, anda second channel providing a passage between first and second ends ofthe main channel, whereby optical fluid may flow in the second channelbetween the first and second ends of the main channel as said mirrormoves therein.
 18. The scannable mirror of claim 16 wherein saidsubstrate includes an optical glass having an index of refraction, andwherein the optical fluid has an index of refraction greater than theindex of refraction of the optical glass.
 19. The scannable mirror ofclaim 16 wherein said substrate includes an optical glass, and whereinsaid optical fluid includes a silicone fluid and/or benzene.
 20. Thescannable mirror of claim 16 wherein the channel of said substrate has acounterbore at the first end thereon and wherein said optical fiber isdisposed in the counterbore of the channel.
 21. The scannable mirror ofclaim 15 wherein the optical waveguide has a cross-sectional shape andsize and wherein said mirror has a different cross-sectional shapeand/or a different cross-sectional size.
 22. The scannable mirror ofclaim 21 wherein the cross-sectional shape of said optical waveguide isone of rectangular, trapezoidal and circular, and wherein thecross-sectional shape of said mirror is circular.
 23. The scannablemirror of claim 15 wherein said optical fiber has a cross-sectionalshape and size and wherein said mirror has about the samecross-sectional shape and size.
 24. The scannable mirror of claim 15wherein said motor means includes a stepping motor, a linear motor, atranslating motor, an electromagnetic motor and/or an electrostaticmotor.
 25. The scannable mirror of claim 15 wherein said motor meansincludes a magnet moveable along and adjacent the optical waveguidetoward and away from the end of said optical fiber, and wherein saidmirror includes a magnetic and/or ferromagnetic member.
 26. Thescannable mirror of claim 15 wherein said motor means includes aplurality of electrodes spaced apart along and proximate the opticalwaveguide, and wherein said mirror includes a dielectric member having aplurality of spaced apart electrodes thereon.
 27. The scannable mirrorof claim 26 wherein the plurality of electrodes on said substrate arespaced apart at a pitch greater than that of the plurality of electrodeson the dielectric member of said mirror.
 28. The scannable mirror ofclaim 15 wherein said optical fiber is a multimode optical fiber andwherein the optical waveguide is a fluid-filled multimode opticalwaveguide.
 29. A scannable mirror arrangement comprising: an opticalwaveguide in a substrate adapted for receiving an optical fiber at afirst end of the optical waveguide; a mirror disposed in the opticalwaveguide facing the first end thereof and moveable therein toward andaway from the first end thereof; and a motor moving said mirror in theoptical waveguide toward and away from the first end of said opticalwaveguide, whereby the distance light travels in the optical waveguidebetween the first end thereof and said mirror changes in response tomoving said mirror.
 30. The scannable mirror arrangement of claim 29wherein said optical waveguide is filled with an optical fluid, and/orwherein said motor is an electromagnetic and/or an electrostatic motor.